get_klass_info should handle proxy models and with select_related

[milosu]

If I do not apply attached patch to django/db/models/query.py, the following test case fails:

======================================================================
ERROR: test_defer (modeltests.defer.tests.DeferTests)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "C:\Python26\lib\site-packages\django_versions\Django-1.4c1\tests\modeltests\defer\tests.py", line 149, in test_defer
    for obj in ChildProxy.objects.all().select_related().only('id'):
  File "C:\Python26\lib\site-packages\django_versions\Django-1.4c1\django\db\models\query.py", line 118, in _result_iter
    self._fill_cache()
  File "C:\Python26\lib\site-packages\django_versions\Django-1.4c1\django\db\models\query.py", line 875, in _fill_cache
    self._result_cache.append(self._iter.next())
  File "C:\Python26\lib\site-packages\django_versions\Django-1.4c1\django\db\models\query.py", line 290, in iterator
    requested=requested, only_load=only_load)
  File "C:\Python26\lib\site-packages\django_versions\Django-1.4c1\django\db\models\query.py", line 1276, in get_klass_info
    load_fields.update(fields)
AttributeError: 'NoneType' object has no attribute 'update'

----------------------------------------------------------------------
Ran 170 tests in 0.700s

FAILED (errors=1)

[milosu]

patch and test case

[Simon Charette]

I can confirm the attached testcase fails with 1.4-rc-1 but pass with 1.3.1 thus marking as Release blocker.

[Karen Tracey]

In [17692]:

Fix #17876: Corrected an exception (regression) raised where select_realted and only is used on a proxy model. Thanks milosu and charettes.

[Przemek Lewandowski <prze.lewandowski@…>]

Patch with improved test.

[Karen Tracey]

In [17695]:

Refs #17876: enhanced new test to actually test underlying function, not just ensure trying to use it does not raise an exception. Thanks Przemek Lewandowski.